Hybrid thin film/thick film solid oxide fuel cell and method of manufacturing the same

ABSTRACT

A SOFC providing higher power densities than PEM-based cells; the possibility of direct oxidation and/or internal reforming of fuel; and reduced SOFC operating temperatures. The SOFC comprises a thin film electrolyte layer. A thick film anode layer is disposed on one surface of the electrolyte layer; and a thick film cathode layer is disposed on the opposite surface of the electrolyte layer. A method of making the SOFC comprises the steps of: creating a well in one side of a dielectric or semiconductor substrate; depositing a thin film solid oxide electrolyte layer on the surface of the well; applying a thick film electrode layer in the electrolyte coated well; creating a counter well in the opposite side of the substrate, the counter well abutting the electrolyte layer; and applying a thick film counter electrode layer in the counter well.

BACKGROUND OF THE INVENTION

[0001] The present invention relates generally to solid oxide fuelcells, and more particularly to such fuel cells having thin filmelectrolytes and thick film electrodes.

[0002] There is considerable current research and industrial activity onthe development of PEM-based fuel cell systems for Micro-Powerapplications. The most common PEM systems proposed would use eitherhydrogen or methanol as a fuel. Hydrogen represents a challenge for fuelhandling and distribution. Methanol may be promising in a DirectMethanol PEM fuel cell, but a reduction to commercial practice has notbeen demonstrated to date. Further, methanol has a relatively low(approximately one half) specific energy as compared to otherhydrocarbon fuels such as, for example, butane, propane, gasoline, anddiesel. Reported power densities from PEM cells seldom exceed 400mW/cm2.

[0003] Solid Oxide fuel cells (SOFC) have been shown to offer thepotential for internal reforming, as well as reported power densities ashigh as 1900 mW/cm2. A schematic representation of an SOFC is shown inFIG. 1, wherein V₀ ^(oo) stands for oxygen vacancy. The oxygen reductionreaction (taking place at the cathode) is:

O₂+4e ⁻→2O²⁻.

[0004] The O²⁻ ion is transferred from the cathode through theelectrolyte to the anode. Some typical fuel oxidation reactions (takingplace at the anode) are:

2H₂+2O²⁻→2H₂O+4e ⁻  (1)

2CO+2O²⁻→2CO₂+4e ⁻  (2)

[0005] The oxidation reaction at the anode, which liberates electrons,in combination with the reduction reaction at the cathode, whichconsumes electrons, results in a useful electrical voltage and currentthrough the load.

[0006] The application of “thin film” processing techniques has beenreported to reduce the practical operating temperature of SOFC from arange of 800° C. to 1100° C., down to about 500° C. or less.

[0007] It has also generally been believed that a “thin” electrolytelayer should not be too thin, and thicknesses less than 10 μm have beendiscouraged in order to avoid the possibility of short circuiting. Someresearchers have attempted to provide an improved colloidal depositiontechnique over the prior technique—prior attempts to use colloidaldeposition to deposit films thicker than 10 μm in a single step coatinghad previously resulted in cracking of the film after drying.

[0008] The “thin” film SOFCs are not, however, the SOFCs having thehighest demonstrated performance to date. The higher performance/higherpower density SOFCs are generally operated at higher temperatures, anduse cermets and thick film processes for anode and cathode fabrication.These high performance SOFCs use “thin” film electrolytes; however,these “thin” film electrolytes generally have thicknesses of about 40 μmor more and are fabricated by electrochemical vapor deposition (EVD),tape casting, and other ceramic processing techniques.

[0009] A known thin film SOFC 100 is shown in FIG. 2. SOFC 100 comprisesa substrate 102 having thereabove a nitride layer 104, a thin filmnickel anode 106, a thin film electrolyte 108, and a thin film silvercathode 110.

[0010] Some previously known SOFCs have been electrolyte supported(wherein the electrolyte layer provided some structural integrity andwas thicker than either the anode or the cathode); cathode supported(wherein the cathode layer provided some structural integrity and wasthicker than either the anode or the electrolyte); or anode supported(wherein the anode layer provided some structural integrity and wasthicker than either the cathode or the electrolyte).

[0011] Fabrication has generally been recognized to be one of the majorproblems inherent with SOFC. This is due to the fact that all of thecomponents (anode, cathode, electrolyte, interconnect material, etc.)should be compatible with respect to chemical stability and mechanicalcompliance (eg. thermal expansion coefficients). The layers also shouldbe deposited such that suitable adherence is achieved without degradingthe material due to use of too high a sintering temperature. Theserequirements have heretofore rendered successful and cost effectiveproduction of high performance SOFCs very difficult.

[0012] Thus, it would be desirable to provide a SOFC and method offabricating a SOFC which overcome the above-mentioned drawbacks.

SUMMARY OF THE INVENTION

[0013] The present invention addresses and solves the above-mentionedproblems and meets the objects and advantages enumerated hereinbelow, aswell as others not enumerated, by providing a fuel cell, preferably asolid oxide fuel cell, comprising a thin film electrolyte layer having afirst surface and a second surface, the first surface being opposed tothe second surface. A thick film anode layer is disposed on the firstsurface; and a thick film cathode layer is disposed on the secondsurface.

[0014] A method of making the fuel cell of the present inventioncomprises the step of creating a well in one side of a dielectric orsemiconductor substrate. A thin film solid oxide electrolyte layer isdeposited on the surface of the well. An electrode layer is applied inthe electrolyte coated well. A counter well is created in the other sideof the substrate, the counter well abutting the electrolyte layer. Themethod further comprises the step of applying a counter electrode layerin the counter well.

BRIEF DESCRIPTION OF THE DRAWINGS

[0015] Other objects, features and advantages of the present inventionwill become apparent by reference to the following detailed descriptionand drawings, in which:

[0016]FIG. 1 is a schematic diagram of a basic solid oxide fuel cellstructure;

[0017]FIG. 2 is a cross sectional view of a prior art thin film solidoxide fuel cell structure;

[0018]FIG. 3 is a cross sectional view of a preliminary step in theprocess of the present invention, showing a masking film on both sidesof the substrate;

[0019]FIG. 4 is a cross sectional view of a further step in the presentprocess, showing the masking film patterned on one side of thesubstrate;

[0020]FIG. 5 is a cross sectional view of a further step in the presentprocess, showing a well formed in the substrate material;

[0021]FIG. 6 is a cross sectional view of a further step in the presentinvention, showing the masking film removed from the substrate adjacentthe well;

[0022]FIG. 7 is a cross sectional view of a further step in the presentinvention, showing the application of the thin solid electrolyte layer;

[0023]FIG. 8 is a cross sectional view of a further step in the presentinvention, showing the application of a thick film electrode in thewell;

[0024]FIG. 9 is a cross sectional view of a further step in the presentinvention, showing the masking film patterned on the opposite side ofthe substrate;

[0025]FIG. 10 is a cross sectional view of a further step in the presentinvention, showing a counter well formed in the opposite side of thesubstrate material;

[0026]FIG. 11 is a cross sectional view of a further step in the presentinvention, showing the masking film removed from the substrate adjacentthe counter well;

[0027]FIG. 12 is a cross sectional view of a further step in the presentinvention, showing an isolation dielectric on the substrate adjacent thecounter well;

[0028]FIG. 13 is a cross sectional view of a further step in the presentinvention, showing the application of a thick film counter electrode inthe counter well;

[0029]FIG. 14 is a semi-schematic top view of the invention shown inFIG. 13, depicting anode and cathode contact pads;

[0030]FIG. 15 is a cutaway cross sectional view of a planar array ofseveral of the SOFCs of the present invention; and

[0031]FIG. 16 is an electrical schematic diagram of the planar arrayshown in FIG. 15.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0032] It is an object of the present invention to provide a solid oxidefuel cell with a thin film electrolyte in combination with both a thickfilm anode/fuel electrode and a thick film cathode/air electrode;thereby advantageously achieving lower operating temperatures and higherperformance/power densities. It is a further object of the presentinvention to provide a method for producing such a solid oxide fuelcell, which method advantageously incorporates process steps from themicro-electronics industry and is efficient and cost effective. Yetfurther, it is an object of the present invention to provide anintegrated planar array of such a thin film/thick film solid oxide fuelcell, which planar array advantageously provides a simplified means fortailoring operating voltages of a fuel cell system.

[0033] It has been unexpectedly and fortuitously discovered in thepresent invention that, in sharp contrast to conventional SOFC, the SOFCof the present invention may exhibit high performance (eg. higher powerdensities than conventional PEM cells, and perhaps higher powerdensities than conventional high performing SOFC) at lower operatingtemperatures. Lower operating temperatures are quite desirable in thatless expensive materials may be utilized as components of the SOFC. As ageneral rule, as the operating temperature rises, the more expensive theSOFC component materials become. However, conventionally (as discussedabove) in order to take advantage of lower operating temperatures,higher performance had to be sacrificed.

[0034] Without being bound to any theory, it is believed that theinventive SOFC successfully achieves high performance at lower operatingtemperatures through the combination of thick film electrode materialsto a thin film electrolyte. It is to be understood that “thin film”within the context of the present invention is defined to encompassthicknesses generally associated with the electronics/semiconductorindustry, ie. thicknesses achievable with processes such as sputterdeposition, for example from less than 1 μm to about 20 μm. Suchthicknesses for a “thin” film SOFC electrolyte, although recognized inthe literature, have heretofore not been reduced to commercial practice.

[0035] Thus, the SOFC of the present invention is a “hybrid” in thesense that the thin film electrolyte is formed by processes which havetraditionally been used in the micro-electronics industry; eg. in thefabrication of integrated circuits; while the thick film electrodes areformed by traditional SOFC fabrication techniques. Some examples ofthese traditional SOFC processes include, but are not limited to PowderPress & Sinter, Powder Extrusion & Sinter, Colloid Suspension Spray orDip Coating, Screen Printing, Slurry Method, Tape Casting, TapeCalendering, Plasma Spray Coating, Flame Spray Coating & SprayPyrolysis, Electrochemical Vapor Deposition (EVD), Chemical VaporDeposition (CVD), and the like.

[0036] It is to be understood that not all of these traditional thickfilm SOFC fabrication techniques may be suitable for use in the presentinvention. In the preferred embodiment, any desired thick filmelectrodes may be applied by processes including, but not limited toColloid Suspension Spray or Dip Coating, Screen Printing, Slurry Method,Plasma Spray Coating, Flame Spray Coating & Spray Pyrolysis, andChemical Vapor Deposition (CVD).

[0037] Referring now to FIG. 13, the hybrid thin film/thick film solidoxide fuel cell of the present invention is designated generally as 10.The solid oxide fuel cell (SOFC) 10 comprises a thin film electrolytelayer 12 having a first surface 14 and a second surface 16, the firstsurface 14 being opposed to the second surface 16. A thick film anodelayer/fuel electrode 18 is disposed on the first surface 14; and a thickfilm cathode layer/air electrode 20 is disposed on the second surface16.

[0038] It is to be understood that the thin film electrolyte layer 12may have any thickness as desired and/or suitable for a particular enduse, within the following parameters. The electrolyte layer 12 shouldideally be as thin as possible, yet should be electronically insulating(only ionically conductive), impervious to gases, have enough dielectricstrength to prevent short circuiting of the cell 10, and be thick enoughto cover topographical irregularities thereby providing completeness ofcoverage to also prevent short circuiting of the cell 10.

[0039] In the preferred embodiment, the electrolyte layer 12 may have athickness ranging from about less than 1 micron to about 20 microns. Ina more preferred embodiment, the electrolyte layer 12 may have athickness of less than about 10 microns. In a further preferredembodiment, the electrolyte layer 12 may have a thickness rangingbetween about 2 microns and about 5 microns.

[0040] It is to be understood that each of the thick anode 18 andcathode 20 layers may have any thickness as desired and/or suitable fora particular end use, within the parameters discussed herein. Incontrast to some known SOFCs which promote electrode-supported SOFCswhile keeping the counter electrode thin (as discussed hereinabove), ithas been unexpectedly discovered in the present invention that it wouldbe advantageous if both the anode 18 and the cathode 20 were thick andporous.

[0041] Some of the advantages of thick and porous (an interconnectedporosity) electrodes include, but are not limited to the following. Thethicker the electrode is, the greater the surface area is for desirableelectrocatalytic reactions. This greater surface area, advantageouslypresenting a large three phase boundary area (simultaneous contact ofreactant, electrode catalyst and electrolyte), is especially desirablefor the anode/fuel electrode 18 at which internal reforming (andconsequent production of hydrogen) and/or direct oxidation of fuel takesplace; the larger surface advantageously results in the fuel cell beingable to generate power without being unduly limited by the rate ofproduction of hydrogen. The three phase boundary area is even larger ifthe material chosen for the electrode acts as a Mixed Electronic/IonicConductor (MEIC). Further, the porous thick film electrodes 18, 20 maybe more desirable than known dense thin film electrodes because the fueland oxidant may reach the electrolyte more efficiently (due at least inpart to lower resistance for transport) than with dense thin filmelectrodes. Still further, thicker electrodes offer lower electricalparasitic losses.

[0042] In a preferred embodiment, each of the anode and cathode layershas a thickness greater than about 30 microns. In a further preferredembodiment, each of the anode and cathode layers has a thickness rangingbetween about 30 microns and about 500 microns. It is to be understoodthat, although anode 18 is depicted in FIG. 13 as being thicker thancathode 20, this is a non-limiting example. It is contemplated as beingwithin the scope of the present invention to have an anode 18 andcathode 20 being equal or essentially equal in thickness one to theother, an anode 18 thinner than cathode 20, and so on, provided, howeverthat both electrodes 18, 20 are thick as defined herein (ie. greaterthan about 30 microns).

[0043] The SOFC 10 of the present invention further comprises an anodelayer 18 having an interconnected porosity ranging between about 19% andabout 55%; and the cathode layer 20 has an interconnected porosityranging between about 19% and about 55%. In a more preferred embodiment,each of the anode layer 18 interconnected porosity and the cathode layer20 interconnected porosity ranges between about 20% and about 25%.

[0044] The chosen materials for the anode and/or the cathode (thematerials are discussed in further detail hereinbelow) may be renderedwith an interconnected porosity by any conventionally known process. Anon-limitative example of such a process is to mix a suitable poreforming material, such as starches; suitable binders or polymers; andsuitable solvents to form a ceramic paste/slurry. Then, in a two stepthermal process, the binder and solvents are driven off, and the poreformer is oxidized at high temperatures. Then, the material is sinteredat temperatures typically greater than 1000° C., achieving solid statediffusion and the consolidation of the ceramic and/or metallicparticles. This renders a material having an interconnected porosity. Asis well known in the art, various process parameters may be varied inorder to render a particular percentage of interconnected porosity.

[0045] It is to be understood that many suitable materials may be chosenfor the various layers 12, 18, 20 (as well as for the interconnectionand interfacial materials discussed hereinbelow). In the preferredembodiment, the electrolyte layer 12 comprises a material selected fromthe group consisting of yttria stabilized zirconia (YSZ) (between about8 mol % and about 10 mol % Y₂O₃), samaria doped ceria (SDC, one exampleof its stoichiometric composition being Ce_(0.8)Sm_(0.2)O_(1.9)),partially stabilized zirconia (PSZ), stabilized bismuthsesquioxide(Bi₂O₃), tantalum pentoxide (Ta₂O₅), and lanthanum strontium galliummagnesium oxide (LSGM, one example of its stoichiometric compositionbeing La_(0.8)Sr_(0.2)Ga_(0.85)Mg_(0.15)O_(2.825)).

[0046] In a more preferred embodiment, the electrolyte layer 12 consistsessentially of Ta₂O₅ or lanthanum strontium gallium magnesium oxide(LSGM). In an alternate preferred embodiment, the electrolyte layer 12consists essentially of YSZ or SDC.

[0047] In the preferred embodiment, the anode layer 18 comprises amaterial selected from the group consisting of nickel (Ni), Ni-yttriastabilized zirconia cermet (Ni-YSZ cermet), copper doped ceria,gadolinium doped ceria, strontium doped ceria, yttria doped ceria,Cu-YSZ cermet, Co-stabilized zirconia cermet, Ru-stabilized zirconiacermet, LSGM+nickel oxide, and mixtures thereof.

[0048] In the preferred embodiment, the cathode layer 20 comprises amaterial such as silver or the like, or a material having a perovskitestructure. In the preferred embodiment, the cathode layer 20 comprises amaterial having a perovskite structure selected from the groupconsisting of lanthanum strontium manganate (LSM), lanthanum strontiumferrite, lanthanum strontium cobaltite (LSC), LaFeO₃/LaCoO₃, YMnO₃,CaMnO₃, YFeO₃, and mixtures thereof. LSC and LSM are more preferredcathode materials; while Ag is suitable but less preferred.

[0049] It is to be understood that either the cathode layer 20 and/orthe anode layer 18 may be formed from a material which serves as a MixedElectronic/Ionic Conductor (MEIC).

[0050] The fuel cell 10 of the present invention may further comprise afirst interfacial layer 22, positioned between the anode 18 and theelectrolyte 12; and a second interfacial layer 24 positioned between thecathode 20 and the electrolyte 12. It is to be understood that theinterfacial layers 22, 24 may comprise any suitable materials which,desirably, provide buffering and/or interdiffusion barrier properties aswell as serving as Mixed Electronic/Ionic Conductors (MEIC). In thepreferred embodiment, the first interfacial layer 22 comprises yttriadoped ceria (YDC, one example of its stoichiometric composition being(Y₂O₃)_(0.15)(CeO₂)_(0.85))), and the second interfacial layer 24comprises yttria stabilized bismuthsesquioxide (YSB, Bi₂O₃).

[0051] It is to be understood that the interfacial materials for layers22, 24 may or may not be interchangeable. For example, Yttria DopedCeria (YDC) has been reported to be used as a buffer at both theanode/electrolyte and the cathode/electrolyte interfaces.

[0052] The SOFC 10 of the present invention may further comprise amaterial 26, 26′ for connecting the fuel cell 10 to an electrical load Land/or an electrical storage device (not shown), the connecting material26, 26′ deposited on at least one of the anode layer 18 and the cathodelayer 20. It is to be understood that connecting layer 26, 26′ may covera portion of, or substantially all of the surface of the anode 18 and/orcathode 20. Layer 26, 26′ may also cover a portion of, or substantiallyall of the electrolyte layer 12 on one opposed surface 40 of thesubstrate 30, and it may also cover a portion or substantially all ofthe isolation dielectric layer 46 on the other opposed surface 42 of thesubstrate 30. However, it is contemplated that if layer 26, 26′ extendsbeyond the surface of the anode 18 and/or the cathode 20, the processfor fabricating fuel cell 10 may need more than two masks (the processand masks 48, 50 are discussed further hereinbelow).

[0053] The electrical load L may comprise many devices, including butnot limited to any or all of computers, portable electronic appliances(eg. portable digital assistants (PDAs), portable power tools, etc.),and communication devices, portable or otherwise, both consumer andmilitary. The electrical storage device may comprise, as non-limitativeexamples, any or all of capacitors, batteries, and power conditioningdevices. Some exemplary power conditioning devices includeuninterruptable power supplies, DC/AC converters, DC voltage converters,voltage regulators, current limiters, etc. It is also contemplated thatthe SOFC 10 of the present invention may be suitable for use in thetransportation industry, eg. to power automobiles, and in the utilitiesindustry, eg. within power plants.

[0054] It is to be understood that the connecting material 26, 26′ maycomprise any suitable material, however, in the preferred embodiment,this connecting material has as a main component thereof a materialselected from the group consisting of silver, palladium, platinum, gold,titanium, tantalum, chromium, iron, nickel, carbon, and mixturesthereof.

[0055] SOFC 10 may further comprise a material 28, 28′ forinterconnecting at least two of the hybrid thin film/thick film solidoxide fuel cells 10 (a planar array of cells 10 is shown in FIG. 15),the interconnecting material 28, 28′ deposited on at least one of theanode layer 18 and the cathode layer 20.

[0056] The interconnecting material 28, 28′ may be any suitablematerial. However, in the preferred embodiment, this material 28, 28′ isselected from the group consisting of lanthanum chromites, nickel,copper, titanium, tantalum, chromium, iron, carbon, and mixturesthereof.

[0057] It is to be understood that the materials for the connectinglayer 26, 26′ may or may not be interchangeable with the materials forinterconnecting layer 28, 28′.

[0058] Some additional materials which could be used as connectingmaterials 26, 26′ and/or interconnecting materials 28, 28′ include butare not limited to W (tungsten), stainless steels (if the operatingtemperatures are reduced enough), and high temperature nickel alloys,eg. some such alloys are commercially available under the tradenamesINCONEL 600 and INCONEL 601 from International Nickel Company inWexford, Pa., and HASTELLOY X and HA-230 from Haynes International, Inc.in Kokomo, Ind.

[0059]FIG. 14 is a semi-schematic top view of the fuel cell 10 shown inFIG. 13, showing anode contact pad 56 and cathode contact pad 58.

[0060] It is to be understood that any suitable fuel/reactant may beused with the SOFC 10 of the present invention. In the preferredembodiment, the fuel/reactant is selected from the group consisting ofmethane, butane, propane, pentane, methanol, ethanol, higher straightchain or mixed hydrocarbons (preferably low sulfur hydrocarbons, eg. lowsulfur gasoline, low sulfur kerosine, low sulfur diesel), and mixturesthereof. In a more preferred embodiment, the fuel/reactant is selectedfrom the group consisting of butane, propane, methanol, pentane, andmixtures thereof. Suitable fuels should be chosen for their suitabilityfor internal and/or direct reformation, suitable vapor pressure withinthe operating temperature range of interest, and like parameters.

[0061] It is contemplated as being within the purview of the presentinvention that a large number of fuel cells 10 may be formed by variouscombinations of the listed materials for layers 12, 18, 20. A largernumber of fuel cells 10 may be formed by various combinations of thelisted materials for layers 12, 18, 20 with any or all of the optionallayers 22, 24, 26, 28. It is to be understood that such “mixing andmatching” is within the scope of the present invention; however, it ispreferred that the following guidelines be followed. It is preferredthat there be mechanical compatibility between the chosen layers, eg.the layers should have substantially matched thermal coefficients. It isalso preferred that there be chemical compatibility between the chosenlayers, eg. there should be a lack of undesirable reactions duringfabrication at elevated temperatures, there should be a lack ofundesirable reactions in use, etc. It is further preferred that thechosen layers perform in the operating temperature range of interest.Further, it is preferred that the fuel(s) chosen perform within theoperating temperature of interest.

[0062] Referring now to FIG. 15, an additional aspect of the presentinvention comprises a plurality of the hybrid thin film/thick film fuelcells 10, 10′, 10″ arrayed within a substrate 30. An electricalconnection (either series and/or parallel) is provided between theplurality of anode layers 18; and an electrical connection (eitherseries and/or parallel) is provided between the plurality of cathodelayers 20.

[0063]FIG. 15 depicts a preferred embodiment of the array, wherein theplurality of fuel cells 10, 10′, 10″ are connected within a planar array32, the planar array 32 having a first plane 34 adapted to contact asource of oxygen, the first plane 34 having a plurality of cathodelayers 20 therein. The planar array 32 further has a second plane 36opposed to the first plane 34, the second plane 36 adapted to contact afuel (not shown), the second plane 36 having a plurality of anode layers18. In the preferred embodiment, the source of oxygen is air.

[0064]FIG. 16 is an electrical schematic diagram of the planar array 32shown in FIG. 15.

[0065] The planar array 32 shown in FIG. 15 may be fabricated using a 7mask process. Some advantages of planar array 32 include, but are notlimited to the following. By fabricating a planar array in the mannershown, a complete practical series and/or parallel combination of cells(to obtain the desired output voltage/current operating characteristics)for a micropower application can be constructed on a single substrate,using common and established microelectronic fabrication techniques.This translates into a probable economic advantage over traditional3-dimensional stacking approaches, as well as providing a great deal ofdesign flexibility. Further, the planar array results in a simplifiedfuel and air manifolding system as a result of the anodes and cathodesall being on their own common side of a single substrate, as opposed tothe configuration of a 3-dimensional stack. This simplified manifoldingwould eliminate the need for expensive bipolar plates and elaborate gassealing schemes. Sealing problems have proven to be a substantialdrawback for many known planar SOFC concepts. A lower electricalparasitic loss would be likely as a result of simplified interconnectionbetween individual cells, as well as a simplified connection to theexternal load.

[0066] The fuel cell 10 of the present invention is high performing, andhas quite desirable power densities. In a preferred embodiment, SOFC 10has a power density of between about 100 mW/cm² and greater than about2000 mW/cm². In a more preferred embodiment, the fuel cell 10 has apower density of between about 1000 mW/cm² and about 2000 mW/cm².

[0067] The fuel cell 10 preferably has an operating temperature ofbetween about 400° C. and about 800° C. More preferably, the fuel cell10 has an operating temperature of between about 400° C. and about 600°C. Still more preferably, the fuel cell 10 has an operating temperatureof between about 400° C. and about 500° C.

[0068] The present invention advantageously provides power densitiesapproximately 2 to 10 times that of PEM-based cells; the possibility ofdirect oxidation and/or internal reforming of fuel; and reduced SOFCoperating temperatures.

[0069] A method of making the fuel cell 10 of the present inventioncomprises the step of creating a well 38 in a dielectric orsemiconductor substrate 30, the substrate 30 having a first side 40 anda second side 42, the second side 42 opposed to the first side 40, andthe well 38 being defined in the first side 40 (see FIG. 5). A thin filmsolid oxide electrolyte layer 12 is deposited on the surface of the well38 (see FIG. 7). An electrode layer 18 is applied in the electrolyte 12coated well 38 (see FIG. 8). A counter well 44 is created in the secondside 42, the counter well 44 abutting the electrolyte layer 12 (see FIG.10). The method of the present invention further comprises the step ofapplying a counter electrode layer 20 in the counter well 44 (see FIG.13).

[0070] It is to be understood that, although the “electrode” isdesignated as anode layer 18, and the “counter electrode” is designatedas cathode layer 20, these may be reversed; ie. “electrode” may becathode layer 20, and “counter electrode” may be anode layer 18.

[0071] It is to be understood that the thin film electrolyte layer 12may be deposited by any suitable means, however, in the preferredembodiment, the step of depositing the electrolyte layer 12 is performedby sputter deposition and/or chemical vapor deposition (CVD).

[0072] The method of the present invention may optionally furthercomprise the step of firing the electrolyte layer 12 prior toapplication of the electrode layer 18. This step may or may not benecessary. For example, if the electrolyte layer 12 can be sputterdeposited at a high enough temperature, the firing step may beunnecessary. Further, a step of firing the electrodes 18, 20 may sufficefor the electrolyte layer 12 also, thus rendering a separate electrolyte12 firing step unnecessary.

[0073] The method of the present invention further comprises the step ofapplying/depositing an isolation dielectric 46 on the second side 42 ofthe substrate 30 (see FIG. 12). Further, if the chosen substrate 30 issilicon, the isolation dielectric 46 may be grown on the second side 42of the substrate 30.

[0074] It is to be understood that any suitable material may be chosenfor the isolation dielectric 46; however, in the preferred embodiment,the isolation dielectric 46 material is selected from the groupconsisting of thermally grown silicon dioxide, plasma enhanced chemicalvapor deposited (PECVD) silicon dioxide, PECVD silicon nitride, PECVDsilicon carbide, low pressure chemical vapor deposited (LPCVD) siliconnitride, and mixtures thereof. In the preferred embodiment, the materialof choice is thermally grown silicon dioxide, which is aself-masking/self-aligning oxide. In contrast, the deposited films maygenerally require the use of an additional masking level.

[0075] The method of the present invention may further comprise the stepof processing the electrode layer 18 and the counter electrode layer 20using planarization techniques. It is to be understood that any suitableplanarization techniques may be used; however, in the preferredembodiment, the planarization is performed by chemical mechanicalpolishing (CMP) and/or mechanical polishing. The planarization is amethod for advantageously confining the electrode layer 18 and thecounter electrode layer 20 to the well 38 and the counter well 44,respectively. It is to be further understood that planarizationprocessing of anode material 18 may be completed either before or afterfiring, or after a low temperature consolidation thermal step. Likewise,it is to be understood that planarization processing of cathode material20 may be completed either before or after firing, or after a lowtemperature consolidation thermal step.

[0076] The method of the present invention may further comprise the stepof applying/depositing a hard mask 48 to the first side 40 of thesubstrate 30 before the step of creating a well 38. The method of thepresent invention may also further comprise the step ofapplying/depositing a hard mask 50 to the second side 42 of thesubstrate 30 before the step of creating a counter well 44. If thesubstrate 30 is silicon (as depicted in the Figures), the hard masks 48,50 may be grown on the substrate 30 first and second sides 40, 42. It isto be understood that any suitable masks 48, 50 may be used; however, inthe preferred embodiment, the masks 48, 50 are selected from the groupconsisting of oxides, nitrides, carbides, and mixtures thereof. In amore preferred embodiment, the masks 48, 50 are selected from the groupconsisting of silicon oxides, silicon nitrides, silicon carbides, andmixtures thereof. Although less preferred, masks 48, 50 may comprisemetallic hard masks.

[0077] It is to be understood that the well 38 and counter well 44 maybe formed by any suitable means, including but not limited to etchingand pressing. Pressing could generally be considered, for example, if amaterial such as alumina were chosen as the substrate 30, and if thesubstrate 30 were fabricated by pressing and sintering. In this case,well 38 could be formed by pressing during the substrate fabricationprocess.

[0078] In the preferred embodiment, the well 38 and counter well 44 arecreated by etching. If the substrate 30 is silicon, the etching maypreferably be performed by an etchant selected from the group consistingof wet anisotropic etchants, plasma anisotropic etchants, and mixturesthereof. These etchants advantageously form ultra-smooth surfaces on thewell 38 and the counter well 44.

[0079] It is to be understood that any suitable wet anisotropic etchantsmay be used, provided that they form the ultra-smooth surfaces asdescribed herein. In the preferred embodiment, the wet anisotropicetchants are selected from the group consisting of potassium hydroxide(KOH), tetramethyl ammonium hydroxide (TMAH), a mixture of potassiumhydroxide and isopropyl alcohol, ammonium hydroxide, sodium hydroxide,cerium hydroxide, ethylene diamine pyrocatechol, and mixtures thereof.The wet anisotropic etchants advantageously form side walls 52, 54 ofwell 38, counter well 44 at opposed, outwardly extending angles,substantially as shown in the Figures (see, for example, FIGS. 5 and10). These angular side walls 52, 54 may be advantageous for thermalexpansion/contractions reasons.

[0080] Likewise, it is to be understood that any suitable plasma (dry)anisotropic etchants may be used, provided that they form theultra-smooth surfaces as described herein. In the preferred embodiment,the plasma anisotropic etchant is an alternating application of sulfurhexafluoride, then C₄F₈. The C₄F₈ leaves a thin polymeric film on theetched surface, and especially on the side walls. The application ofsulfur hexafluoride, then C₄F₈ is repeated until the desired etch isachieved.

[0081] The plasma anisotropic etchants may be desirable in that they arecapable of forming very deep wells 38, 44. However, the plasmaanisotropic etchants also form substantially vertical (not shown) sidewalls 52, 54, which may in some instances be undesirable for thermalexpansion/contraction reasons, as well as for side wall coverage ofelectrolyte 12 and electrode 18, 20 materials (ie. it is difficult tocoat substantially vertical walls).

[0082] In a less preferred embodiment, an isotropic etchant may be usedon a silicon substrate 30. It is to be understood that any suitableisotropic etchant may be used; however, in the preferred embodiment, theisotropic etchant is a mixture of hydrofluoric acid, nitric acid andacetic acid. The isotropic etchants provide a curvilinear etch havingsemi-circular cross sections, however, a drawback is that the mask(s)may get undesirably undercut by the isotropic etchant.

[0083] If the substrate is a silicon oxide containing dielectricsubstrate, it is to be understood that the etching may be performed byany suitable isotropic etchant. In the preferred embodiment, theisotropic etchant comprises a hydrofluoric containing isotropic etchant.

[0084] It is to be understood that any suitable material for substrate30 may be chosen. In the preferred embodiment, the substrate 30 isselected from the group consisting of single crystalline silicon,polycrystalline silicon, silicon oxide containing dielectric substrates,alumina, sapphire, ceramic, and mixtures thereof. Single crystal siliconis the substrate of choice in the preferred embodiment of the presentinvention.

[0085] It has unexpectedly and fortuitously been discovered by thepresent inventor that these ultra-smooth surfaces obtained byfabrication processes traditionally used in the micro-electronicsindustry allow for deposition of a very thin film electrolyte layer 12,substantially without risk of surface irregularities causing undesirableopenings in the electrolyte layer 12.

[0086] In contrast, known SOFC fabrication processes deposit anelectrolyte layer on a porous electrode. However, when a porouselectrode is the substrate, there may be an uneven surface for theelectrolyte layer, and there may be some invasion of the electrolytematerial into the electrode as it is deposited. This may produce anuneven electrolyte layer, and often may require a thicker electrolytelayer to ensure that there is no gap in the electrolyte for air, fuel orgases to seep through.

[0087] There are further advantages from the method of the presentinvention. The electrolyte layer 12 is deposited (before either of theelectrodes 18, 20) on a substantially non-porous substrate 30 (eg. awafer of single crystal silicon) over the above-mentioned ultra-smoothwell/counter well 38,44 surfaces. In addition to allowing for depositionof very thin electrolyte layers 12, it is believed that the ultra-smoothsurfaces and the substantially non-porous substrate 30 may result inopen-circuit voltages (OCV) close to theoretical values.

[0088] It is contemplated as being within the purview of the method ofthe present invention to form thin film electrodes 18, 20 within wells38, 44, while retaining many, but not all of the advantages of the SOFCof the present invention. If such thin film electrodes 18, 20 aredesired, they may be applied by any suitable technique, including butnot limited to chemical vapor deposition or sputter deposition. As such,the well 38 and/or the counter well 44 may be adapted to contain eithera thick film or a thin film electrode layer 18, 20. In one of thepreferred embodiments of the present invention, wells 38, 44 are eachadapted to contain thick film electrodes 18, 20.

[0089] Referring now to FIG. 4, the method of the present invention mayadditionally comprise the step of patterning hard mask 48 on the firstside 40 of substrate 30, using conventional photolithography and etchprocesses. FIG. 6 depicts dielectric hard mask 48 removed. Such removalis preferably accomplished by a one side plasma etch.

[0090] After application of electrode 18 into well 38, the electrode 18may be fired. Likewise, after application of electrode 20 into well 44,the electrode 20 may be fired.

[0091] Referring now to FIG. 9, the method of the present invention mayadditionally comprise the step of patterning hard mask 50 on the secondside 42 of substrate 30, using conventional photolithography and etchprocesses. FIG. 11 depicts dielectric hard mask 50 removed. Such removalis preferably accomplished by a one side plasma etch. Mask 50 may beleft behind if desired.

[0092] Some further advantages of the present invention include, but arenot limited to the following. The method of the present invention mayadvantageously be as low as a 2 mask process (as is depicted in FIGS.3-13); whereas state of the art microprocessors generally use about a≧25 mask process. The use of higher performance anode/cathode materialsfrom porous thick film media as set forth hereinabove lead to lesspolarization loss. The SOFC 10 of the present invention, as well asplanar array 32 of the present invention provide for layout flexibilityas well as scaleable layout schemes. Further, the process steps asdescribed hereinabove do not need to progress in the exemplary order setforth—the inventive processing sequence may be advantageously alteredand flexible, based upon etch selectivities and thermal historyconstraints. Further, since fuel cell 10 allows the opportunity forinternal reforming reactions (which convert a hydrocarbon fuel tohydrogen and carbon monoxide), this advantageously allows for adiversity of fuel sources.

[0093] To further illustrate the present invention, the followingexample is given. It is to be understood that this example is providedfor illustrative purposes, and is not to be construed as limiting thescope of the present invention.

EXAMPLE

[0094] The SOFC 10 of the present invention is fabricated using thefollowing materials. La+Sr+Ga+Mg+O (LSGM)+NiO Cermet is chosen for theanode layer 18. An anode/electrolyte interfacial layer 22 is formed fromSm+Ce+O (SDC). La+Sr+Ga+Mg+O (LSGM) is chosen for the electrolyte layer12. La+Sr+Co+O (LSC) is chosen for the cathode layer 20. This example ofSOFC 10 is a low operating SOFC, with operating temperatures betweenabout 600° C. and about 800° C.

[0095] While preferred embodiments of the invention have been describedin detail, it will be apparent to those skilled in the art that thedisclosed embodiments may be modified. Therefore, the foregoingdescription is to be considered exemplary rather than limiting, and thetrue scope of the invention is that defined in the following claims.

What is claimed is:
 1. A fuel cell, comprising: a thin film electrolyte layer having a first surface and a second surface, the first surface being opposed to the second surface; a thick film anode layer disposed on the first surface; and a thick film cathode layer disposed on the second surface.
 2. The fuel cell as defined in claim 1 wherein the electrolyte layer has a thickness ranging from about less than 1 micron to about 20 microns.
 3. The fuel cell as defined in claim 2 wherein the electrolyte layer has a thickness of less than about 10 microns.
 4. The fuel cell as defined in claim 3 wherein the electrolyte layer has a thickness ranging between about 2 microns and about 5 microns.
 5. The fuel cell as defined in claim 1 wherein each of the anode and cathode layers has a thickness greater than about 30 microns.
 6. The fuel cell as defined in claim 5 wherein each of the anode and cathode layers has a thickness ranging between about 30 microns and about 500 microns.
 7. The fuel cell as defined in claim 1 wherein the anode layer has an interconnected porosity ranging between about 19% and about 55%; and the cathode layer has an interconnected porosity ranging between about 19% and about 55%.
 8. The fuel cell as defined in claim 7 wherein each of the anode layer interconnected porosity and the cathode layer interconnected porosity ranges between about 20% and about 25%.
 9. The fuel cell as defined in claim 1 wherein the electrolyte layer comprises a material selected from the group consisting of yttria stabilized zirconia, samaria doped ceria, partially stabilized zirconia, stabilized bismuthsesquioxide (Bi₂O₃), tantalum pentoxide (Ta₂O₅), and lanthanum strontium gallium magnesium oxide.
 10. The fuel cell as defined in claim 9 wherein the electrolyte layer consists essentially of Ta₂O₅.
 11. The fuel cell as defined in claim 9 wherein the electrolyte layer consists essentially of lanthanum strontium gallium magnesium oxide.
 12. The fuel cell as defined in claim 1 wherein the anode layer comprises a material selected from the group consisting of nickel (Ni), Ni-yttria stabilized zirconia cermet, copper doped ceria, gadolinium doped ceria, strontium doped ceria, yttria doped ceria, Cu-YSZ cermet, Co-stabilized zirconia cermet, Ru-stabilized zirconia cermet, LSGM+nickel oxide, and mixtures thereof.
 13. The fuel cell as defined in claim 1 wherein the cathode layer comprises a material having a perovskite structure.
 14. The fuel cell as defined in claim 13 wherein the cathode layer comprises a material selected from the group consisting of lanthanum strontium manganate, lanthanum strontium ferrite, lanthanum strontium cobaltite, LaFeO₃/LaCoO₃, YMnO₃, CaMnO₃, YFeO₃, and mixtures thereof.
 15. The fuel cell as defined in claim 1 wherein the cathode layer comprises silver.
 16. The fuel cell as defined in claim 1, the fuel cell further comprising: a first interfacial layer, positioned between the anode and the electrolyte; and a second interfacial layer positioned between the cathode and the electrolyte.
 17. The fuel cell as defined in claim 16 wherein the first interfacial layer comprises yttria doped ceria (YDC).
 18. The fuel cell as defined in claim 16 wherein the second interfacial layer comprises yttria stabilized bismuthsesquioxide (YSB).
 19. The fuel cell as defined in claim 1, further comprising a material for connecting the fuel cell to at least one of an electrical load and an electrical storage device, the connecting material deposited on at least one of the anode layer and the cathode layer.
 20. The fuel cell as defined in claim 19 wherein the electrical load comprises at least one of computers, portable electronic appliances, and communication devices.
 21. The fuel cell as defined in claim 19 wherein the electrical storage device comprises at least one of capacitors, batteries, and power conditioning devices.
 22. The fuel cell as defined in claim 19 wherein the connecting material has as a main component thereof a material selected from the group consisting of silver, palladium, platinum, gold, titanium, tantalum, chromium, iron, nickel, carbon, stainless steels, high temperature nickel alloys, tungsten, and mixtures thereof.
 23. The fuel cell as defined in claim 1, further comprising a material for interconnecting at least two of the fuel cells, the interconnecting material deposited on at least one of the anode layer and the cathode layer.
 24. The fuel cell as defined in claim 23 wherein the interconnecting material is a material selected from the group consisting of lanthanum chromites, nickel, copper, titanium, tantalum, chromium, iron, carbon, stainless steels, high temperature nickel alloys, tungsten, and mixtures thereof.
 25. The fuel cell as defined in claim 1, further comprising: a plurality of the fuel cells arrayed within a substrate; an electrical connection between the plurality of anode layers; and an electrical connection between the plurality of cathode layers.
 26. The fuel cell as defined in claim 25 wherein the plurality of fuel cells are connected within a planar array, the planar array having a first plane adapted to contact a source of oxygen, the first plane having a plurality of cathode layers therein, the planar array further having a second plane opposed to the first plane, the second plane adapted to contact a fuel, the second plane having a plurality of anode layers therein.
 27. The fuel cell as defined in claim 26 wherein the source of oxygen is air.
 28. The fuel cell as defined in claim 26 wherein the fuel is selected from the group consisting of methane, butane, propane, pentane, methanol, ethanol, higher straight chain or mixed hydrocarbons, and mixtures thereof.
 29. The fuel cell as defined in claim 1 wherein the fuel cell has a power density of between about 100 mW/cm² and about 2000 mW/cm².
 30. The fuel cell as defined in claim 29 wherein the fuel cell has a power density of between about 1000 mW/cm² and about 2000 mW/cm².
 31. The fuel cell as defined in claim 30 wherein the fuel cell has an operating temperature of between about 400° C. and about 800° C.
 32. The fuel cell as defined in claim 31 wherein the fuel cell has an operating temperature of between about 400° C. and about 600° C.
 33. The fuel cell as defined in claim 32 wherein the fuel cell has an operating temperature of between about 400° C. and about 500° C.
 34. A fuel cell, comprising: a thin film electrolyte layer having a first surface and a second surface, the first surface being opposed to the second surface, wherein the electrolyte layer has a thickness of less than about 10 microns; a thick film anode layer disposed on the first surface, wherein the anode layer has an interconnected porosity ranging between about 19% and about 55%; and a thick film cathode layer disposed on the second surface, wherein the cathode layer has an interconnected porosity ranging between about 19% and about 55%, and wherein each of the anode and cathode layers has a thickness greater than about 30 microns.
 35. The fuel cell as defined in claim 34 wherein each of the anode layer, cathode layer and electrolyte layer are formed substantially within a substrate.
 36. The fuel cell as defined in claim 35 wherein the electrolyte layer comprises a material selected from the group consisting of yttria stabilized zirconia, samaria doped ceria, partially stabilized zirconia, stabilized bismuthsesquioxide (Bi₂O₃), tantalum pentoxide (Ta₂O₅), and lanthanum strontium gallium magnesium oxide; wherein the anode layer comprises a material selected from the group consisting of nickel (Ni), Ni-yttria stabilized zirconia cermet, copper doped ceria, gadolinium doped ceria, strontium doped ceria, yttria doped ceria, Cu-YSZ cermet, Co-stabilized zirconia cermet, Ru-stabilized zirconia cermet, LSGM+nickel oxide, and mixtures thereof; and wherein the cathode layer comprises a material selected from the group consisting of lanthanum strontium manganate, lanthanum strontium ferrite, lanthanum strontium cobaltite, LaFeO₃/LaCoO₃, YMnO₃, CaMnO₃, YFeO₃, silver, and mixtures thereof.
 37. The fuel cell as defined in claim 35, the fuel cell further comprising: a first inter-facial layer, positioned between the anode and the electrolyte; and a second interfacial layer positioned between the cathode and the electrolyte.
 38. The fuel cell as defined in claim 37, further comprising a material for connecting the fuel cell to at least one of an electrical load and an electrical storage device, the connecting material deposited on at least one of the anode layer and the cathode layer, wherein the electrical load comprises at least one of computers, portable electronic appliances, and communication devices, and wherein the electrical storage device comprises at least one of capacitors, batteries, and power conditioning devices.
 39. The fuel cell as defined in claim 38, further comprising a material for interconnecting at least two of the fuel cells, the interconnecting material deposited on at least one of the anode layer and the cathode layer.
 40. The fuel cell as defined in claim 39, further comprising: a plurality of the fuel cells arrayed within a substrate; an electrical connection between the plurality of anode layers; and an electrical connection between the plurality of cathode layers.
 41. The fuel cell as defined in claim 40 wherein the plurality of fuel cells are connected within a planar array, the planar array having a first plane adapted to contact a source of oxygen, the first plane having a plurality of cathode layers therein, the planar array further having a second plane opposed to the first plane, the second plane adapted to contact a fuel, the second plane having a plurality of anode layers.
 42. A method of making a fuel cell, the method comprising the steps of: creating a well in a dielectric or semiconductor substrate, the substrate having a first side and a second side, the second side opposed to the first side, and the well being defined in the first side; depositing a thin film solid oxide electrolyte layer on the surface of the well; applying an electrode layer in the electrolyte coated well; creating a counter well in the second side, the counter well abutting the electrolyte layer; and applying a counter electrode layer in the counter well.
 43. The method as defined in claim 42 wherein the step of depositing the electrolyte layer is performed by at least one of sputter deposition and chemical vapor deposition (CVD).
 44. The method as defined in claim 42, further comprising the step of firing the electrolyte layer prior to application of the electrode layer.
 45. The method as defined in claim 42, further comprising the step f applying an isolation dielectric on the second side of the substrate.
 46. The method as defined in claim 42 wherein the substrate is silicon, and wherein the isolation dielectric is grown on the second side of the substrate.
 47. The method as defined in claim 42, further comprising the step of processing the electrode layer and the counter electrode layer using planarization techniques.
 48. The method as defined in claim 42 wherein the planarization is performed by at least one of chemical mechanical polishing (CMP) and mechanical polishing.
 49. The method as defined in claim 42, further comprising the step of applying a hard mask to the first side of the substrate before the step of creating a well.
 50. The method as defined in claim 49 wherein the substrate is silicon, and wherein the first side hard mask is grown on the substrate first side.
 51. The method as defined in claim 42, further comprising the step of applying a hard mask to the second side of the substrate before the step of creating a counter well.
 52. The method as defined in claim 51 wherein the substrate is silicon, and wherein the second side hard mask is grown on the substrate second side.
 53. The method as defined in claim 42 wherein the step of creating the well and the step of creating the counter well are each carried out by etching.
 54. The method as defined in claim 42 wherein the substrate is silicon, and wherein the etching is performed by an etchant selected from the group consisting of wet anisotropic etchants, plasma-anisotropic etchants, and mixtures thereof, thereby forming ultra-smooth surfaces on the well and the counter well.
 55. The method as defined in claim 54 wherein the wet anisotropic etchants are selected from the group consisting of potassium hydroxide (KOH), tetramethyl ammonium hydroxide (TMAH), a mixture of potassium hydroxide and isopropyl alcohol, ammonium hydroxide, sodium hydroxide, cerium hydroxide, ethylene diamine pyrocatechol, and mixtures thereof.
 56. The method as defined in claim 54 wherein the plasma anisotropic etchant is sulfur hexafluoride alternated with C₄F₈.
 57. The method as defined in claim 42 wherein the substrate is a silicon oxide containing dielectric substrate, and wherein the etching is performed by a hydrofluoric containing isotropic etchant.
 58. The method as defined in claim 42 wherein the substrate is selected from the group consisting of single crystalline silicon, polycrystalline silicon, silicon oxide containing dielectric substrates, alumina, sapphire, ceramic, and mixtures thereof.
 59. The method as defined in claim 58 wherein the substrate is single crystalline silicon.
 60. The method as defined in claim 42 wherein at least one of the well and the counter well is adapted to contain a thick film electrode layer.
 61. The method as defined in claim 60 wherein the well contains a thick film electrode and the counter well contains a thick film counter electrode.
 62. A method of making a fuel cell, the method comprising the steps of: creating a well in a dielectric or semiconductor substrate, the substrate having a first side and a second side, the second side opposed to the first side, and the well being defined in the first side; depositing a thin film solid oxide electrolyte layer on the surface of the well, wherein the step of depositing the electrolyte layer is performed by at least one of sputter deposition and chemical vapor deposition (CVD); applying an electrode layer in the electrolyte coated well; creating a counter well in the second side, the counter well abutting the electrolyte layer, wherein the step of creating the well and the step of creating the counter well are each carried out by etching; applying an isolation dielectric on the second side of the substrate; applying a counter electrode layer in the counter well; and processing the electrode layer and the counter electrode layer using planarization techniques, wherein the planarization is performed by at least one of chemical mechanical polishing (CMP) and mechanical polishing.
 63. The method as defined in claim 62, further comprising the step of firing the electrolyte layer prior to application of the electrode layer.
 64. The method as defined in claim 62 wherein the substrate is silicon, and wherein the isolation dielectric is grown on the second side of the substrate.
 65. The method as defined in claim 62, further comprising the step of applying a hard mask to the first side of the substrate before the step of creating a well.
 66. The method as defined in claim 65 wherein the substrate is silicon, and wherein the first side hard mask is grown on the substrate first side.
 67. The method as defined in claim 62, further comprising the step of applying a hard mask to the second side of the substrate before the step of creating a counter well.
 68. The method as defined in claim 67 wherein the substrate is silicon, and wherein the second side hard mask is grown on the substrate second side.
 69. The method as defined in claim 62 wherein the substrate is silicon, and wherein the etching is performed by an etchant selected from the group consisting of wet anisotropic etchants, plasma anisotropic etchants, and mixtures thereof, thereby forming ultra-smooth surfaces on the well and the counter well.
 70. The method as defined in claim 69 wherein the wet anisotropic etchants are selected from the group consisting of potassium hydroxide (KOH), tetramethyl ammonium hydroxide (TMAH), a mixture of potassium hydroxide and isopropyl alcohol, ammonium hydroxide, sodium hydroxide, cerium hydroxide, ethylene diamine pyrocatechol, and mixtures thereof.
 71. The method as defined in claim 69 wherein the plasma anisotropic etchant is sulfur hexafluoride alternated with C₄F₈.
 72. The method as defined in claim 62 wherein the substrate is a silicon oxide containing dielectric substrate, and wherein the etching is performed by a hydrofluoric containing isotropic etchant.
 73. The method as defined in claim 62 wherein the substrate is selected from the group consisting of single crystalline silicon, polycrystalline silicon, silicon oxide containing dielectric substrates, alumina, sapphire, ceramic, and mixtures thereof.
 74. The method as defined in claim 73 wherein the substrate is single crystalline silicon.
 75. The method as defined in claim 62 wherein at least one of the well and the counter well is adapted to contain a thick film electrode layer.
 76. The method as defined in claim 75 wherein the well contains a thick film electrode and the counter well contains a thick film counter electrode. 